Appealing to the public for help with funding the legal challenge, Mr Maugham, director of the Good Law Project, said: “This is a case vital in our national interest. Please help us engage the best possible team.”

The European Court of Justice will consider if the UK can unilaterally withdraw its Article 50 letter in November (Victoria Jones/PA)

The case has been brought by Mr Maugham and a cross-party group of politicians in Scotland, comprising Labour MEPs Catherine Stihler and David Martin, Joanna Cherry MP and Alyn Smith MEP of the SNP, and Green MSPs Andy Wightman and Ross Greer.